Higher operating temperatures for gas turbine engines are continuously sought in order to increase efficiency. However, as operating temperatures increase, the high temperature durability of the components within the engine must correspondingly increase. In this regard, materials comprising silicon, particularly those with silicon carbide (SiC) as a matrix material or a reinforcing material, are considered useful for high temperature applications, such as for combustor and other hot section components of gas turbine engines.
However, some silicon substrates may recede and lose mass as a result of formation of volatile Si species, particularly Si(OH)x and SiO when exposed to high temperature, aqueous environments, thus necessitating the use of a protective coating thereon. Accordingly, methods such as described in U.S. Pat. Nos. 5,985,470, 6,444,335, 6,410,148 and 6,759,151, the contents of each of which are incorporated by reference, have addressed shortcomings concerning the use of such silicon substrates by providing an environmental barrier coating (EBC) over the substrate. The EBCs inhibit formation of volatile silicon species, Si(OH)x and SiO, thereby reducing recession and mass loss. A thermal barrier coating (TBC) typically comprising yttria stabilized zirconia may also be employed as an outer layer to the EBC depending upon the operating conditions employed.
While the current state of the art EBCs, which are typically multi-layer EBCs, may be effective in preventing water vapor recession of the ceramic matrix composite (CMC) substrate, both BSAS, SiC and some rare earth silicates such as some silicates disclosed in U.S. Pat. No. 6,759,151 may be susceptible to silicate glass formation when in contact with sulfate salt deposits as operating conditions continue to increase.
Accordingly, there is a need to reduce the silicate glass formation rate in materials comprising silicon, particularly in EBC materials. Embodiments of the present invention satisfy this need and others.